Microelectromechanical Systems for Wireless Radio Front-ends and Integrated Frequency References
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Microelectromechanical systems (MEMS) have great potential in realizing chip scale devices for energy efficient analog spectrum processing. This thesis presents the integrated development of a new class of MEMS resonators and filters integrated with CMOS readout circuits for RF front-ends and integrated timing applications. RF MEMS devices are fabricated using a surface micromachined integrated passive device (IPD) process. Using this process, a miniaturized ultra-wideband (UWB) filter has been demonstrated. To further address the issue of narrow in-band interferences in UWB communication, a tunable MEMS bandstop filter is integrated with the bandpass filter with more than an octave frequency tuning range.
In the second part of the thesis, an aluminum nitride (AlN) thin-film process is added to the platform for realizing piezoelectric MEMS. Fused silica is explored as a new structural material for fabricating vibrating micromechanical resonators. A piezoelectric-on-silica MEMS resonator is demonstrated with high Q value (Q > 20,000) and excellent electromechanical energy coupling. A low phase noise CMOS reference oscillator is implemented using the MEMS resonator. Temperature-stable operation of the MEMS oscillator is realized by ovenizing the platform using an integrated heater. In an alternative scheme, the intrinsic frequency drift of MEMS is utilized for temperature sensing, and active compensation for MEMS oscillators is realized by oven-control using a phase-locked loop (PLL). The active compensation technique realizes a MEMS oscillator with an overall frequency drift within +/- 4 ppm across a working temperature range of -40 degrees C to70degrees C without the need for calibration. CMOS circuits are implemented for realizing the PLL-based low-power oven-control system, showing near-zero phase noise invasion on the MEMS oscillators.